Xev creates a window and then asks the
X server to send it events whenever anything happens to the window (such
as it being moved, resized, typed in, clicked in, etc.). You can also attach
it to an existing window. It is useful for seeing what causes events to
occur and to display the information that they contain; it is essentially
a debugging and development tool, and should not be needed in normal usage.

This option specifies the size and/or location of the window, if a
window is to be created.

-bw pixels

This option specifies the border width
for the window.

-bs {NotUseful,WhenMapped,Always}

This option specifies what
kind of backing store to give the window. The default is NotUseful. Backing
store refers to the the pixels saved off-screen when the X server maintains
the contents of a window; NotUseful means that the xev process will redraw
its contents itself, as necessary.

-id windowid

This option specifies that
the window with the given id should be monitored, instead of creating a
new window.

-s

This option specifies that save-unders should be enabled on
the window. Save unders are similar to backing store, but they refer rather
to the saving of pixels off-screen when the current window obscures other
windows. Save unders are only advisory, and are normally set for popup dialogs
and other transient windows.